4 Description and components of Gravel filters

Page 1 of 33 Gravel Filters - Agricultural

FILTRATION MODULE

SECTION 5

GRAVEL FILTRATION (AGRICULTURAL)

Netafim School of Irrigation - Copyright


Table of Contents

1

2

3

4

5

6

7

Table of Contents

Introduction

Objectives

Background on the development of Gravel filters

Description and components of Gravel filters

4.1 Components of Gravel Filters

The process of filtration and backflushing

5.1 Gravel Filters – the Principle

5.2 The Filtration Process

5.3 The Backflush Process

5.4 When to back flush

Automatic batteries

Selection and Design

7.1 Pre-filtration

7.2 General Selection

8

9

Installation and Operation

Maintenance

9.1 Gravel Filters

9.2 Backup Filters

10 Applications

11 Summary and Conclusion

12 Acknowledgements

13 Review Questions

13.1 Beginner

13.2 Intermediate

13.3 Advanced

17

19

19

20

24

26

26

27

28

30

30

31

31

32

33

2

3

4

5

5

6

14

14

15

16

17



1 Introduction

Water sources for micro-irrigation include municipal water, reservoirs, dams, lakes, rivers, channels and bores. Solids contaminants include inorganic matter such as sand, silt, clay, aquatic growth such as algae, fish and snails, vegetation such as stubble trash, weeds and leaves and other such contaminants including plastic bags, paper and bottles.

As global water quality is deteriorating and water quantity is becoming restrictive as well as mounting economic pressure, agricultural producers need to be utilizing their resources as efficiently as possible. Gravel filtration is one tool used to increase their operating efficiency by improving and maintaining the efficiency of their irrigation systems.

The use of irrigation in agriculture requires water to be filtered. Filtration for agricultural uses serves the purpose of protecting irrigation systems from clogging and/or abrasion. Therefore, its purpose is to stop only those particles which may cause clogging or abrasion from entering the irrigation system.

It is necessary to consider the various water sources and the changes that occur within the sources over time in order to select the correct filtration system that suits the irrigation system, operator and budget.

This section being Gravel Filters (Agricultural) will cover all aspects of

 The development of gravel filters

 Description and components of gravel filters



The process of filtration and backflush

 Automatic gravel filter batteries



Selection and design



Installation and operation

 Maintenance



Applications

These areas will be covered in depth with the use of examples where possible and be followed by a series of review questions.



2 Objectives

On completion of this module, Gravel Filters (Agricultural), participants should be able to: -

 List all components of a gravel filter and describe ea ch component’s role in both single tank systems and batteries

 Understand automatic batteries in terms of operation, application and function

 Understand the filtration and backflush process for a single tank manual system and an automatic battery

 Design and select gravel filters for a given application

 Understand the parameters and gravel filter selection criteria

 Understand how to operate single tank or automatic batteries

 Understand how to install single tank or automatic batteries

 Understand how to maintain single tank or automatic batteries

 Improve knowledge of the applications of gravel filtration systems



3 Background on the development of Gravel filters

Media filtration has been occurring since the beginning of time. Ever since rain and flooding water seeped through the earth into the crystal clear aquifers media filtration was occurring.

In terms of agriculture, in the late 1960’s, early 1970’s, gravel tanks were being used. Odis filtration began developing filters in Israel, carbon steel with a protective coating. They were then similar in style as some of those used today.

Automation of these filter tanks soon followed and in the mid 1970’s the first stainless steel filters were used. In 1993 Arkal started to produce the Arkal

Gravel Filter (AGF) - the only plastic filter widely used in the marketplace today.

Since those early days a tremendous amount of improvements have occurred.

The design of the “mushrooms”, the shape of the diffuser plates, the backflush valve changed from two valves into one, the controllers have changed and even simple changes such as easy to remove inspection lids.

Research and development has been ongoing which has led to the exciting development of today’s gravel filter systems.

4 Description and components of Gravel filters

Filters in general can be broken into two main types when looking at units that are widely used in agriculture, surface filtration and volume filtration. Firstly surface filtration. Screen filters are single surface, two dimensional filters. They have only one retention point for solids and all apertures of the screen are usually of uniform size. These two facets greatly limit the success of this type of filtration therefore limiting screen filters to roles such as back up filters to gravel systems or infield backup filters. Secondly, volume filtration (depth filtration) that consists of gravel filters and disc filters. Both types of filters have multi dimensional layers of filtration enabling far better efficiency and safety in terms of collecting organic or inorganic material.



4.1 Components of Gravel Filters

There are many components that compose the hardware of a gravel filtration system. Whether the filtration system is a single tank system or a multi-tank battery, the same components apply but naturally are increased in number in accordance to the number of tanks. Refer Fig 1 below

View tube

Air valve

Pressure relief valve

Command filter

Restriction valve

Vacuum breaker

Inlet manifold

Backwash manifiold

Filter body

Backwash valve

Outlet manifold

Fig 1

Pressure gauge

Media filter body

– The filter body or tank is a receptacle that contains the gravel/sand media. It is a vertical tank consisting of one or more external inspection or access ports, a top inlet and a bottom outlet. Three legs usually support it.

Internally the filter body contains a diffuser plate, located just under the inlet. This diffuser plate is designed to spread the incoming water over the filter media.

Located above the outlet, also internal to the filter body is the under-drain. The under-drain has three purposes.

 To allow clean water to exit the filter tank

 To prevent the filter media from exiting the filter tank along with the clean water



 It distributes the backflush water evenly throughout the filter bed during the backflush process.

This under-drain, in the case of ODIS consists of a welded, reinforced steel plate that divides the tank into an upper and lower chamber. This plate is covered with

“mushroom” diffusers that are plastic, slotted conical cups that collect the filtered water and also evenly disperse the backflush water uniformly in the flushing process. ARKAL AGF gravel filters use a series of upper and lower diffuser arms radiating out from the centre drum. These diffuser arms have the “mushrooms” evenly spaced along each one. (Refer Fig 2 below) These arms or flutes are specifically designed so that the orifice under the mushrooms is larger the further they are away from the central hub. This is to allow even backflushing across the media bed. The main problem with other types of filter tanks is that they are unable to backflush evenly across the bottom of the bed or are unable to backflush close to or against the tank walls. These factors lead to residual dirt and foreign material being kept in the tank that builds up over time and grips onto the gravel. This causes a blockage that forces the water through a smaller area under higher velocity. This process forms tunnels or channels through the gravel media causing poorer filtration. This also makes backflushing less effective.

Fig 2

Slots

Mushroom or diffuser



Filter tanks can be either coated carbon steel, reinforced polythene or stainless steel type 304 and 316. ODIS filter tanks are carbon steel that have a 100 micron protective coating of extra durable polyester applied electrostatically and oven cured on a zinc phosphate layer. ARKAL AGF filter tanks have an outer layer made from reinforced polyester with an inner layer of black polythene. Type 304 stainless steel is sometimes used; it is corrosion resistant but not rust proof. Rust is a definite concern for SS304 if water salinity is above 750 ppm; some pitting has been reported at even lower salt levels.

Both ODIS and ARKAL filter bodies have upper and lower inspection ports.

These are easy to remove covers that allow for visual inspection, addition or removal of gravel.

ODIS gravel tanks are a vailable in body diameters of 12”, 16”, 20”, 24”, 36” and

48”. They have a maximum working pressure of 10 bar.

ARKAL AGF tanks are only available in 48” and are rated to 6 bar. Higher ratings are available upon request.

The gravel media is the critical factor in term of filtration efficiency. Netafim recommends (in order of preference).

Crushed basalt #1

Crushed granite #11

Crushed silica #12, #16 or #20

The particles of sand should be crushed ensuring the grains have sharp edges and are not round.

For surface and ditch water that contain a high level of algae and organic materials 70 –75% of the media particles should be between the sizes of 1.23 –

2.25 mm. The other 30% can be outside those tolerances, but no smaller than

0.8 mm or no larger that 2.6mm.

For water with high levels of iron, the size of the media should be smaller; 70 –75

% should be between the sizes of 0.75 – 1.75 mm. The other 30% can be outside those tolerances, but no smaller that 0.6 mm and not bigger than 2.0 mm.



The life of the gravel is directly proportional to the hours of filtration and the level of abrasives in the water. Usually 4 - 5 years can be expected.

Media Mesh

Micron (µ)

Min

Crushed silica #20 170

–230 88 – 60

0.3

Crushed silica #16 155 –200 95 - 70

Equivalent Silica Size (mm)

0.5

Max

0.5

0.8

Average

0.4

0.65

Crushed silica #12 80 - 130 170 – 120 0.8 1.3 1.05

0.75 Crushed basalt #1 140 -180 105 - 80

Table 1

0.4

Filtration Media (ex Arkal)

1.1

ODIS and ARKAL AGF filter tanks have level indicator marks located externally for easy recognition of the correct media level.

Gravel filter manifolds consist of the inlet manifold (brings unfiltered water into the filter body), outlet manifold (takes filtered water to the mainline) and the backflush manifold (takes flush water and debris away from the filter system).

Ideally the backflush manifold has a visible, above water level exit point, so a visual appraisal of backflushing can be done. A transparent viewing tube can also be used in this instance. The exit point of the backflush manifold should not exit at any point close to the footvalve. Ideally the backflush pipe should terminate downstream or quite some distance from the footvalve. Care must be taken not to run the backflush pipe too long in terms of distance or have a pipe with a diameter that is too small. The pipe diameter should be able to cater for the backflush flow with less than 5 metres /100 metres loss. The higher the loss the shorter the backflush pipe should be. 50 metres are about the maximum length possible. It is possible to see and set backflush times from this information.

These manifolds can be manufactured with galvanized steel, stainless steel, and epoxy coating or made entirely of polythene. The corrosive nature of the water chemistry, as well as the application and budget will determine the most practical manifold material for each application. Manifold diameters are sized in accordance to maximum flow rate so as to give a minimum friction loss across


Page 10 of 33 Gravel Filters - Agricultural the filtration system. This friction loss as well as friction loss inside the tank give us the baseline pressure loss i.e. the pressure differential across the filter when it is operating as clean. This baseline pressure loss is usually less than 5 psi. (0.3 bar.)

Most manifolds are modular and connect to valves and filter bodies via victaulic couplings.

Each filter body is equipped with a Backflush Control Valve. The backflush valve connects the top of the filter body to the inlet manifold and the backflush manifold. Its role is to change the direction of the water flow thus changing a filtration mode to a backflushing mode. The backflush valve consists of two built in units, the normally open filter port and the normally closed drain port. Each port has its own seat but share a common stem and diaphragm. The valve operates as a mutually integrated unit, that is when one port is open the other port is closed. This permits the valve to operate both modes. In filtration mode the valve allows unfiltered water to flow from the inlet manifold into the filter tank.

When in backflush mode the inlet port closes and the drain port opens thus reversing the flow of water and allowing filtered water and debris to flow from the filter tank into the backflush manifold via the backflush valve.

This process can be initiated manually or automatically. Manual operation can be initiated by switching the threeway selector, “Sagiv” to the open position. This

Sagiv can be located on the outside of the backflush valve.(Refer Figure 3 below)

A manual bleed screw can also be used to initiate backflushing. Either of these operations will allow water to bleed of the vent command chamber of the valve thus causing the stem and diaphragm to shift into the other mode of operation.

Manual/Semi automatic control of backwash valve

Figure 3



Automatic control of backwash valve

Figure 4

Automatically this same process occurs but is initiated electrically. An electric solenoid or hydraulic relay initiates operation. Water pressure or a pneumatic source then actuates the valve. (Refer Figure 4 above)

Most automatic filter stations have solenoids; AC or DC installed on the backflush valve. A separate Backflush Controller can start a backflush cycle based on pressure differential or time or both. The Backflush Controller is mounted on or near the filtration system and can be of AC or DC operation. It is a stand-alone unit that has the role of automating backflush sequences. The AC power source is converted from 240 volt to 24 volt for the controller. The DC controller relies on a battery that can be trickle charged by solar panel or from an alternator. The backflush controller would be connected to a pressure differential switch that has hydraulic control tubing connecting it to the inlet and outlet manifolds – this then constantly monitors the Pressure Differential (PD).

The backflush controller, PD switches and solenoids are all protected by a

Hydraulic Command Filter. This filter consists of a 20mm up to 40mm disc filter with a 120-mesh element that prevents contaminants from entering the system.

As flow through this filter are very low it is not necessary for this filter to be automated.



Figure 5

The parts shown on Figure 5 above are

1

3

Filter Tank

Outlet manifold

4 Back flush valve

12 Hydraulic Command filter

14 Pressure Gauge – Outlet

Hydraulic Command system

15 Backflush controller

16 Pressure differential switch

17 Victaulic coupling

A check filter or check filters should be supplied and installed on every gravel filter system. The check filter element should be 40 or 80 mesh depending upon the application. The sole purpose of the check filter is to prevent any filtration


Page 13 of 33 Gravel Filters - Agricultural media from entering the irrigation system should a “mushroom” be displaced or damaged. It is purely a precautionary device.

Two choices of check filter design are available. Single check filter, located on end of outlet manifold or multiple check filters, one disc filter per filter tank. Both have advantages and disadvantages. Firstly

 Single check filters . A single check filter is usually a screen filter with a manual ball valve for flushing. This valve is very simple to install and easy to flush to see if gravel is being caught. However, if gravel is being caught it is difficult to see which tank is losing these small amounts of gravel media. The nature of screen filters is also that if the screen builds up with a massive load of gravel or iron etc it can in some circumstances collapse thus contaminating the mainline with the material. Secondly,

 Multiple check filters . In this case one ARKAL check filter is located for every tank. They are slightly harder to check as they have to be undone but they do reveal which tank is yielding gravel media. Blocked ARKAL check filters will unlikely collapse under heavy load but instead reduce in flow until a no flow situation can occur. This can place extra load on the other filters.

A Backflush Restricting Valve needs to be installed in the backflush manifold beyond the last filter tank. The flow in the backflush manifold must be restricted in order to prevent gravel media from being flushed out of the gravel tank with the backflush water. In the backflush mode the turbulent action of the water fluidises and suspends the gravel in a boiling type of fashion. Proper adjustment of this valve will allow debris to be flushed out whilst retaining the gravel media inside the tank. Only a small trace of media will be lost through flushing and this is considered optimum. Check to see if media is being washed out. One method is to place your cupped hand or a container under the bottom of the stream as it exits the system. If gravel is present in large amounts the valve needs to be restricted. New gravel does contain many fines. Fill the filter with water to overflowing with a hose to float off the majority of the fines. Then back flush the filter 4 - 6 times to remove fines before adjusting the Back flush Restricting Valve.

A filter isolation valve, pressure gauge and a dual acting air valve are all items that should also be included as part of the filter system. The isolation valve can be used to keep pressure on the filters at start up or simple to shut the system


Page 14 of 33 Gravel Filters - Agricultural down. Pressure gauges are useful tools to check the efficiency of flushing. That it is if after flushing the pressure gauge indicates the baseline pressure you will know that back flushing is very efficient and there is no residual blockage. Air valves are necessary for preventing air locks, preventing water hammer and allowing the system to breath at start up and shut down.

Optional accessories may include Pressure Sustaining Valves, Pressure Relief

Valves, a Check Valve and a Rinse Valve.

Pressure Sustaining Valves can be used to ensure that there is always adequate back pressure on the filters so as to get a proper back flush. The valve operates by hydraulically throttling itself if the downstream pressure decreases past a set point. A minimum pressure of 14 metres (20 psi) needs to be maintained. This type of valve is a must on flat ground and where the filter has irrigation blocks below it.

A Pressure Relief Valve upstream of the filters will protect the filters from over pressure if for example infield valves close and the pump is still operating. At a certain pressure this valve will hydraulically operate, opening to atmosphere and venting to reduce pipe pressure.

A Check Valve should be installed downstream of the filters to prevent any back flow of water once the pump has shut down. Naturally this can be problematic where the field is a lot higher than the filter station. It also allows maintenance on the pump and or filter station without the need for mainline drainage.

Finally, a Rinse Valve can be used to initially clean media or simply act as a source of water at the pump and filter station.

5 The process of filtration and backflushing

5.1 Gravel Filters – the Principle

Gravel filters operate by allowing unfiltered water to pass through a bed of aggregate that captures the suspended material as the water passes through - all inside a pressurized tank. This media captures the debris that would normally


Page 15 of 33 Gravel Filters - Agricultural jeopardize the integrity of the emitters in the field. The bed of aggregate recommended is crushed basalt, has many varying aperture sizes that makes the water travel through many passages on its way from the inlet to the outlet of the filter. Due to the large volume and contact area between the water and the media/gravel particles, various physical forces occur, thus retaining the contaminant particles. Efficiency of large particle retention is very high as long as the gravel is kept clean.

5.2 The Filtration Process

During the filtration process, unfiltered water enters the gravel filter system via the inlet manifold, cleaned and discharged via the outlet manifold. During filtration the inlet ports are open and the backflush ports are closed, water flows from top to bottom across the gravel layer. That is water flows over the diffuser plate through the media and into the out the filter bottom through the

“mushrooms”. Refer Figure 6 below

Figure 6

The filtration process



5.3 The Backflush Process

During the backflush process the inlet port closes and the backflush port opens reversing the flow of water in the filter. Clean pressurised water from the other filters now flows into the bottom of the filter, lifting the gravel media and freeing the accumulated dirt and debris that will be then flushed out through the backflush valve and into the backflush manifold. If the backflush restriction valve is adjusted properly, the debris will be flushed out of the filter while the gravel media, being of a higher Bulk Density than the debris will remain fluidised in the filter tank.

Only one filter at a time should backflush. The water that is being used to backflush one filter would have already passed through another filter or filters so that the backflush process is carried out with clean, filtered water.

Refer to Figure 7 below

Figure 7

The backwash process



5.4 When to back flush

Filter flushing (back flushing or back washing) can be carried out either manually or automatically. Whatever the method, flushing should be timed in accordance with one of the following practices.

 Pressure differential

– Backflushing will start whenever the pressure differential across the filter drops to a predetermined level. A pressure differential sensor can sense this level and flushing can be automatically started. This predetermined level of pressure loss is 6 - 8 psi (3 - 6 metres) over the baseline pressure loss. The baseline pressure loss is the measured pressure loss when the filters are clean.

 Time – It is recommended to backflush every 2 - 4 hours regardless of pressure loss.

6 Automatic batteries

A Battery or Filter Array simply consists of multiple filter tanks. This can mean 2 or 50 tank systems. More often than not batteries are automated so as to save labour and maximise efficiency.

At least two tanks should be used. In a two-tank system, half of the water flows through each tank. In a three tank system one third of the water flows through each tank. Only one tank is back flushed at one time and it uses clean water from the other tank(s) for the back flush. Tanks are back flushed in sequence at a time set interval or when there is a large enough pressure differential. With two tank systems especially, problems can arise where there is not enough flow or back pressure for adequate flushing or interruption to flows into the field occur when the filter goes into back flush. What occurs is all of the flow going through one tank, to give clean water to back flush the other, puts the filtering tank into a high flow situation. This can cause huge friction losses and channeling of the tank media. Channeling is the term used when excessive velocities in a filter tank cause the water to force “worm holes” through the media. These worm holes or tracks cause filtration efficiencies to be reduced as actual contact filtering surface


Page 18 of 33 Gravel Filters - Agricultural area is reduced. It is therefore advisable to design a filtration system with at least two tanks, preferable three or more. Ideally, back flushing a battery of filters, because only one filter is back flushed at one time, can occur without interfering with irrigation. In a back flush sequence the Dwell Time is the time period between the flushing of each filter. This prevents two filters from flushing at one time through overlap – it allows the valves time to revert to their normal operating position. A usual setting is 15 to 60 seconds.

Three reasons why Filter Batteries are very flexible in terms of design.

 A filter battery can be designed to any flow rate and water condition; it is not limited by size.

 Enlargement- when the project is done in stages, the filter station can be enlarged in stages. In the first stage one installs the number of filters required for the initial flow rate and simply leave vacant, plugged gaps for successive stages.



Flexibility

– when the flow rate and the water quality are not known exactly, the filter battery can be designed to the maximum expected; manifolds, concrete base, controller, etc. The actual array can be larger or smaller with the ease of adding or subtracting tanks as required.

Automatic Filter Batteries can take three main shapes; L, T and W. All have the same filtering capacity but with different manifolding.

“L” shaped batteries are essentially a straight-line system. Unfiltered water enters one end of the line and exits at the opposing end as filtered water.

“T” shaped batteries have the inlet and the outlet at the centre of the battery with the filter tanks being left and right of the inlet and outlet. Manifolds are usually smaller than those of “L” batteries.

“W” shaped batteries tend to resemble to “L” shapes batteries side by side.

Unfiltered water enters one end and exits as filtered water at the far end in two outlets. These batteries are often used for larger numbers of filter tanks in one battery in limited spaces.



7 Selection and Design

Selection and Design should be an objective, rather than a subjective exercise.

Experience, however, does help.

7.1 Pre-filtration

In order for the filtration system to work with the highest degree of success it is highly recommended to install, whenever required, some pre-filtration or screening facility.

 Trash screens, grills made of simple bars or a punched screen act as a good, solid mechanical barrier for holding off large pieces of debris from the pump intake. It should have sufficient area so that the velocity of the water through it is less than 0.3 metres/sec. If there are fish in the water entrance velocities should be less that 0.15 metres/sec.

 In most cases the specific location of the pump foot valve can greatly affect the performance of the filtration system. In large dams the wind direction should be considered. The foot valve should be located if possible up wind, to prevent build up of algae and other vegetation around the valve. The foot valve depth is also important. If it is too close to the bottom silt and clay will be dragged into the system. This situation can become particularly acute towards the end of the season when water levels are falling in the reservoir.

The optimum pumping depth should be between 1.2 and 1.9 metres below the surface. Within this layer, algae are less likely to develop and the concentration of the bio mass is minimal. Foot valves should also be fitted with a screen. This will help keep large contaminants out that and allow more efficient filter operation as well as allowing the foot valve to hold its prime.

 Hydro cyclones (Sand separator) – a sand concentration of 50mg/L indicates the need for this type of device. The hydro cyclone separates sand and other solid matter with very little head loss and 90% or better efficiency. A hydro cyclone can greatly compliment as gravel filtration system in terms of removing a large portion of the sediment load before it reaches the filter tanks.



7.2 General Selection

Selection and Design of the filtration system are critical to the efficiency, longevity and manageability of the entire irrigation system. The selection of the gravel filtration system is based on:

 Irrigation system (sprinklers, micro-sprinklers or drip)

 Required filtration (mesh size or microns)

 Type and quantity of impurities in the water

 Required flow rate (capacity)

 Maximum and minimum water pressure

 Future needs (modification, expansion)

Irrigation System

Prevention of emitter plugging is the main reason for gravel filtration systems.

Micro irrigation systems have relatively small outlets ranging from about 0.35 mm

– 0.85 mm in terms of drip and 0.5mm – 2 mm in terms of micro-sprayers.

Some heuristics exist as to the relationship between the emitter minimum dimension and the filtration level desired. Charles Burt et al (1994) suggests that all particles greater that 1/10 th the diameter of the emission holes should be removed for drip and 1/7 th the orifice diameter of the micro-sprayer nozzle should be sufficient.

The reason for this 1/10 th or 1/7 th rule of thumb is to minimize plugging through

“bridging”. Bridging is a term used when many small particles accumulate together in a passageway and eventually “bridge” the hole.



Required filtration (mesh size or microns)

When talking disc or screen filters the terms mesh, mm and micron are used.

Their relationship is as follows. Refer Table 2 below

Mesh

40

80

120

140

Micron

400

200

130

115 mm

0.4

0.2

0.13

0.12

Table 2

200 55 0.055

600

Litres/Hour

30

– 60

70

– 120

160 – 240

20 0.020

Recommended mesh sizes for micro-sprayers are as follows.

Mesh

80

–100

60

– 80

40 – 60

Generally Netafim drippers require 120-mesh filtration.

In terms of gravel filtration the crushed basalt #1 has a particle size 1.2 - 1.8 mm.

Expected filtration at 50 to 75 m3/hr/m2 equates to 70

–100 microns.

Type and Quantity of impurities in the water

Design should always take into consideration the type and quality of the water source. By selecting the correct filter for the correct size and type of particles that should be removed high retention efficiency and easy cleaning are ensured. This can mean a basic water analysis in most cases. Two main areas are focused on in terms of type and quantity of impurities in the water. These are Solid Mineral materials and Large Micro-organisms. The Solid Mineral Material is primarily soil, that is found in water sources that flood after rains, springs and rivers, in shallow ponds and sand contained in bores. Soils can be lifted into the system via the pump footvalve and vary in terms of type and quantity from site to site, season to


Page 22 of 33 Gravel Filters - Agricultural season and even day to day. Large Micro-organisms include zooplankton and algae that grow in sources that have ample air and light. Throughout the season the quantity of micro-organisms will vary from minor to extreme cases.

Required flow rate (capacity)

The required flow rate has a direct effect on the number of filter tanks selected, and their size. Each filter tank has a given filtration area (and volume for that matter). A flow rate is then applied against this filtration area so as to determine acceptable flow velocities and losses. The results of this process will naturally differentiate as water quality varies from good, average and bad. The following

Table 3, GRAVEL FILTER SELECTION CRITERIA, illustrates the results. In terms of multiple tank systems for larger flows it is simply a matter of extrapolating the figures to arrive at the correct system size.

Maximum and minimum water pressure

Gravel filters need a minimum pressure for filtration and back flushing. This minimum pressure, mostly for backflushing is required so that the valves will operate. This minimum pressure for both ODIS and ARKAL AGF is 15 metres

(22 psi) of pressure during backflush. As mentioned it may be necessary to have installed a pressure-sustaining valve to achieve these minimum pressures during flushing.

Maximum pressures are

 ODIS 100 metres (145 psi or 10 bar) and

 ARKAL AGF 60 metres (85 psi or 6 bar).

Future needs (modification, expansion)

When selecting a filter battery try to have some understanding of any future needs, expansions and modifications that may be planned in successive stages.

In terms of the filters’ hydraulic duty, if there is going to be an increase in flow rate it is possible to plan for this change and allow for modulation of the system.



For example if the initial duty is to be 140 m3/hr in average water and increase yearly by 140 m3/hr for three years thus giving a total future flow of 560 m3/hr the system design would change. That is for 140 m3/hr we could use 4 x 36” or 2 x 48” However the total flow would require 8 x 48” or 14 x 36”. This total flow would make 14 x 36” filters undesirable but be very suitable for 8 x 48”. The filter station could be increased by 2 x 48’ tanks per year. This information is required as a 2 x 48” system has a 150-mm manifold whereas a 8 x 48” system has a

300-mm manifold. The advantage in the above example is that with enough information a 2 x 48” system could be supplied with a 300-mm manifold, with blank inlets and outlets allowing for future tanks to be added as necessary. This planning ahead saves a great deal of time, money and resources.

GRAVEL FILTER SELECTION CRITERIA

MAKE TANK DIAMETER

Table 3

RECOMMENDED FLOW RATE m³/hr

Odis

Inches

12 mm

300

Min Average* Max Backflushing

3.5 5 6 7

Odis

Odis

Odis

Odis

Odis

Odis

Odis

16

20

20

24

30

36

48

400

500

500

600

750

900

1200

6

9

10

14

21

32

62

8

12

12

20

30

42

72

11

18

18

28

42

62

120

Arkal 48 1200 48 65 80 60

Notes



*Good to bad quality (average grade agricultural water)

 For dirtier water reduce the flow rate and/or consult Netafim



Pay attention to required backflush flow rates. It may be necessary to install a pressure-sustaining valve downstream of the filter battery. Correct tank selection needed to suit flow.

10

15

15

25

38

54

95



 Odis standard tanks are rated to 10 bar, Arkal standard tanks rated to 6 bar

 Above flows are on a per tank basis. E.g.

24m³/hr would require 2 x 20” Odis tanks.

Example selection a) Duty 45 m³/hr surface water maximum pressure 75 metres --- Use 3 x 24” tanks b) Duty 60 m³/hr very poor water maximum pressure 40 metres --- Use 3 x

30” or 2 x 36” (Selection 1 is the better option) c) Duty 220 m³/hr surface water maximum pressure 50 metres --- Use 3 x 48”

Odis or 4 x 48 AGF Arkal d) Duty 350 m³/hr sand free bore water (high corrosion) maximum pressure 56 metres – Selection 5 x 48” AGF Arkal (4 x 48” Odis or 6 x 36” Odis would be

OK but due to the corrosive nature of the water the AGF tank would be preferred as would the use of poly manifolds)

8 Installation and Operation

A few points to assist

 Filters should installed on a concrete slab at least 100mm thick.

 The slab should have a slight grade to allow for water run off. But not too great that the manifold will not bolt together easily.

 Grade should not direct water to electrical panel.

 The tanks should be positioned so the access cover is easily accessible for maintenance and inspection i.e. not positioned against a wall.

 Distances between tanks

12” to 16”

20” to 24”

30” to 36”

48” and AGF 48”

600mm

800mm

1000mm

1320mm

 Victaulic Clamp Assembly



Remove bolts from coupling, place gasket onto one of the grooved components to be assembled. Mate the two grooved components and slide the gasket over both so that it is centered between the two grooved fittings place some gasket soap on the seal. Place the clamps around the gasket and tighten bolts evenly.

Assemble all components before full tightening of bolts.

 Backflush Valve Assembly

The valves are assembled on the top of the tank using victaulic clamps. The arrow on the valve should point down towards the filter.

 Adding filtration Media

Before any media is added to any of the tanks first inspect the tanks with the aid of a torch to ensure that there are no foreign objects in the tanks (i.e.: information brochures! ). And ensure that all the mushrooms are installed. In the case of AGF ensure that all the plastic retaining pins that secure the diffusion arms are in place.

The media fill level is indicated on the outside of each tank. When filling care should be taken that the media is evenly spread particularly with 36” and 48” and 48” AGF. A broom can be use to ensure that the media is evenly distributed but care must be taken to ensure that the plastic mushrooms are not damaged.

 Number of 30kg Bags per tank

12” 2 Bags

16” 3 Bags

20” 4 Bags

24” 7 Bags

30” 9 Bags

36” 13 Bags

48” 23 Bags

AGF 48” 23 Bags



After the system has been run for a few hours it should be depressurized and the gravel should be inspected to ensure that the media level is correct and that the media is evenly distributed in the tank.

 System Start up

Prior to pressurization of system

Program Back flush configuration into Back flush controller.

Check that all Back flush solenoids are working electrically.

Ensure that all inspection covers have been tightened up and are secure.

All victaulic clamps have been tightened.

Command water filter is not isolated.

Backflush restriction valve is fully open.

Isolation valve to main line is open.

Pressurization of system

Start pumps and bring system up to operating pressure. Once pressure has been achieved put the 1 st filter into manual Back flush and set restriction valve on the Back flush line to a point where no gravel is being removed from the tanks.

Continue the manual Back flush until each of the tank’s Back flush water is clean.

Note: Only Back flush one tank at a time!

9 Maintenance

Maintenance is an essential yet often overlooked operation

– routine maintenance is simple, inexpensive and cost effective.

9.1 Gravel Filters

Gravel (sand, media) filters are a very simple and reliable filtration system.

These filters generally require minimum servicing. At the end of the irrigation season, remove the covers from the filters. Check the level of media in the tanks and top up if required, manufacturers clearly mark the level of media required in


Page 27 of 33 Gravel Filters - Agricultural each tank (do not over fill the tanks). The media should be loose and level in the tank. If the media appears hard, lumpy and uneven, it is an indication of problems with the filter. The hard media surface should be treated, by mixing the media with a broom handle, to free it up. Several manual flushes, for several minutes may be required after the mixing, to flush the dirt from the filters. In some cases if there are large quantities of algae, the media may have to be treated with chlorine, to oxidize the algae and bacteria build-up from the media.

CHLORINE TREATMENT OF GRAVEL (MEDIA) FILTERS

 Backwash the filters first.



Isolate the filtration system from the irrigation system by closing the isolation valve downstream of the filtration system.

 Fill up the filters with water (using external hose), to above media level. Add four (4) litres of liquid chlorine to each of the filter tanks, and allow to treat for

12 - 24 hours.

 With the chlorine still in the tanks, mix the media thoroughly, to free any solids, which may remain in the tanks.

Note: care must be taken while mixing the media in the tanks, not to damage the internal filtration elements (mushroom) at the bottom of the tanks.

 Manually turn the filters (one at the time), to (ON) Back flush position.

Continue to flush the filter for several minutes, before moving to the next filter.

Continue to flush all filters in the system.

 Open the isolation valve downstream of the filter, start irrigation system, and check the pressure loss across the filter. A clean filter generally, should not lose more then 5 psi (this may vary from one design to another).

 Backup filters should be thoroughly cleaned and serviced at the end of each season. However, in some cases, systems, which are operating with poor water quality, may require more frequent cleaning.

9.2 Backup Filters

Disc Filters



First wash the discs with fresh water

– if scum and sediments persist then



 The discs should be removed from the element (spine) and soaked in an acid or chlorine solution for up to12-24 hours. HCl (Hydrochloric acid) mixed to a diluted solution (1 to 2 litres of acid mixed with 10 litres of water), will remove any buildup of organic and mineral salts from the discs grooves. (Refer to disc

Filter Module).

 The discs should then be sprayed with high-pressure water before installation, to remove any particles and the remains of acid. Damaged or buckled discs should be removed and replaced with new discs (always use the same color discs to replace damaged ones).

 Clean check an d lubricate all “O” rings and moving parts.This acts as both a lubricant and preservative. Replace any damaged “O” rings with new ones.

Note: Use only recommended “O” rings (“EPDM” normally), and silicon based lubricant to lubricate the “O” rings. Eg Molycote 111. Do not use Vaseline® - petroleum based products will corrode the “O” rings.

Screen Filters –

 Screens should be removed and manually cleaned.

 High -pressure water and a soft brush can be used, to remove particles from the screen surface. The screen surface should be checked for damage and tears.

 Gaskets and “O” rings should be checked and replaced, if necessary.

 All “O”rings should be lubricated (silicon based lubricant) prior to installation, as per above.



The filter should be manually flushed at the end of the service, to ensure correct operation.

10 Applications

The main applications of Gravel Filter systems in agriculture are

 Use as a primary filter for water from open water reservoirs and recycled water.

 Most effective form of filtration for water heavily contaminated with algae and organic matter.



 Used as a primary filter for water from bore holes.

 Used in-conjunction with aeration for iron removal (covered in another module)

Gravel media tanks have traditionally been the most popular filter for dirty water applications. They are renowned for removing organic matter and are extremely capable when it comes to high degrees of organic and inorganic material. The depth of the gravel provides 3 dimensional depth filtration enabling it to store much more debris than other filtration types prior to back flush.

Effective back flushing requires large back flushing flows to lift, fluidise the gravel media and cause mass separation of the gravel media and the trapped contaminants. Relative to disc and screen filters gravel systems require a large percentage of backwash water – where the consumer is not sensitive to price, a large footprint and



11 Summary and Conclusion

The subject of gravel filtration in agriculture is broad topic. What we have attempted to do is to gain a broader understanding of gravel filtration in agricultural applications. We have discussed the development of gravel filtration systems and described the components of the gravel filters. We have identified the components and described their role and how they operate. From an understanding of the components of the gravel filters we have covered in depth the processes of filtration and back flushing. We have also identified when and how back flushing should be done. Upon the understanding of the abovementioned processes, we have elaborated on automatic batteries. This has allowed us to develop skills in selection and design for a given duty. Some examples have clarified these skills. As with all mechanical systems installation, operation and maintenance need to be carried out. This has been described in some depth reaffirming our knowledge of gravel filtration. We have then briefly reviewed gravel filtration applications and finalised the module with a series of review questions.

12 Acknowledgements

 Thanks to Mac Ross and Andrew Crawford of Netafim for compiling the bulk of this Section

 Acknowledgment to Arkal and Odis Filtration for information on their equipment



13 Review Questions

The questions are divided in to 3 sections

– Beginner, Intermediate and

Advanced. See below

13.1 Beginner

1) What level do you fill a gravel tank with media?

2) Can filter inlets and outlets be reversed? Why? Why not?

3) What does looking at the backflush water tell you?

4) Name two ways to look at backflush water whilst backflushing?

5) What causes media to exit the backflush manifold?

6) How do you prevent media exiting the backwash manifold?

7) Is inspecting the gravel necessary? What does it tell you?

8) How does gravel media trap algae and dirt?

9) Where would DC control be used compared to AC?

10) What is the minimum back flush time per tank?

11) How often should filters back flush?

12) In automatic systems what determines when filters backflush?

13) What are the maximum pressures Odis and Arkal recommend?

14) How often should gravel filter systems be serviced?

15) What is the role of the check filter? What mesh size is recommended?

16) What options are there for check filters of gravel filters?

17) Why is it necessary to use an air valve on the filter manifold?

18) How do mushrooms work?



13.2 Intermediate

1) What is the procedure if gravel media is found in the check filter?

2) How do you check if all filters are back flushing properly?

3) How long does media last before it needs replacing?

4) When is the use of a pressure sustaining valve necessary?

5) If there has been a power failure will the filters backflush? Why? Why not?

6) What is dwell time? How does it affect the flushing process?

7) What is the relationship between manifolding and filter battery efficiency?

8) What is the baseline pressure?

9) What does baseline pressure have to do with PD setting?

10) Describe the filtration process?

11) Describe the backflush process?

12) What is a diffuser plate? Why is it necessary?

13) Suggest two under drain designs? What are their advantages and disadvantages?

14) How can the position of the foot-valve affect efficiency? Where should it be located?

15) How should a check filter be maintained? How often?

16) What happens if you backwash both filters in a 2 filter battery at the same time?



13.3 Advanced

1) How can gravel filters be used to filter iron? Describe the process?

2) How do you set the back flush control valve? What does it do?

3) What is the maximum distance the back flush pipe can be run from the filters?

What size?

4) What is channeling? How does it occur? How can it be avoided?

5) What is bridging? How does it occur? How can it be prevented?

6) What is better; a 2 tank system or a 3 tank system if the flow rate is the same? Why?

7) What is the importance in the number of back flushes that occur on PD compared to time? How would you find this information?

8) What are the problems associated with over pressure and under pressure?

9) What are the problems associated with over flow and under flow?

10) Why is a water sample beneficial for filter system sizing?

11) What are the advantages and disadvantages of injecting fertilizer before and after the filters?

12) What are the advantages and disadvantages of single check filters and multiple check filters?

13) Can gravel filters be used to remove sand from water?

14) If so will gravel media be removed while flushing the sand out of the tank?

15) Can you mix different types of media in the same tank?


4 Description and components of Gravel filters

FILTRATION MODULE

SECTION 5

GRAVEL FILTRATION (AGRICULTURAL)

Table of Contents

Fig 1

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4 Description and components of Gravel filters

FILTRATION MODULE

SECTION 5

GRAVEL FILTRATION (AGRICULTURAL)

Table of Contents

Fig 1

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